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The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.

Biogenic aerosols critically control atmospheric processes. However, although bacteria constitute major portions of living matter in seawater, bacterial aerosolization from oceanic surface layers remains poorly understood. We analysed bacterial diversity in seawater and experimentally generated aerosols from three Kongsfjorden sites, Svalbard. Construction of 16S rRNA gene clone libraries from paired seawater and aerosol samples resulted in 1294 sequences clustering into 149 bacterial and 34 phytoplankton operational taxonomic units (OTUs). Bacterial communities in aerosols differed greatly from corresponding seawater communities in three out of four experiments. Dominant populations of both seawater and aerosols were Flavobacteriia, Alphaproteobacteria and Gammaproteobacteria. Across the entire dataset, most OTUs from seawater could also be found in aerosols; in each experiment, however, several OTUs were either selectively enriched in aerosols or little aerosolized. Notably, a SAR11 clade OTU was consistently abundant in the seawater, but was recorded in significantly lower proportions in aerosols. A strikingly high proportion of colony-forming bacteria were pigmented in aerosols compared with seawater, suggesting that selection during aerosolization contributes to explaining elevated proportions of pigmented bacteria frequently observed in atmospheric samples. Our findings imply that atmospheric processes could be considerably influenced by spatiotemporal variations in the aerosolization efficiency of different marine bacteria., (© 2015 Society for Applied Microbiology and John Wiley & Sons Ltd.)

This work presents the application of a new method to facilitate the distinction between biologically produced (primary) and atmospherically produced (secondary) organic compounds in ambient aerosols based on their chirality. The compounds chosen for this analysis were the stereomers of 2-methyltetraols, (2R,3S)- and (2S,3R)-methylerythritol, (l- and d-form, respectively), and (2S,3S)- and (2R,3R)-methylthreitol (l- and d-form), shown previously to display some enantiomeric excesses in atmospheric aerosols, thus to have at least a partial biological origin. In this work PM10 aerosol fractions were collected in a remote tropical rainforest environment near Manaus, Brazil, between June 2008 and June 2009 and analysed. Both 2-methylerythritol and 2-methylthreitol displayed a net excess of one enantiomer (either the l- or the d-form) in 60 to 72% of these samples. These net enantiomeric excesses corresponded to compounds entirely biological but accounted for only about 5% of the total 2-methyltetrol mass in all the samples. Further analysis showed that, in addition, a large mass of the racemic fractions (equal mixtures of d- and l-forms) was also biological. Estimating the contribution of secondary reactions from the isomeric ratios measured in the samples (=ratios 2-methylthreitol over 2-methylerythritol), the mass fraction of secondary methyltetrols in these samples was estimated to a maximum of 31% and their primary fraction to a minimum of 69%. Such large primary fractions could have been expected in PM10 aerosols, largely influenced by biological emissions, and would now need to be investigated in finer aerosols. This work demonstrates the effectiveness of chiral and isomeric analyses as the first direct tool to assess the primary and secondary fractions of organic aerosols.

Active vegetation fires in south‐eastern (SE) Europe resulted in a notable increase in the number concentration of aerosols and cloud condensation nuclei (CCN) particles at two high latitude locations—the SMEAR IV station in Kuopio, Finland, and the Zeppelin Observatory in Svalbard, high Arctic. During the fire episode aerosol hygroscopicity κ slightly increased at SMEAR IV and at the Zeppelin Observatory κ decreased. Despite increased κ in high CCN conditions at SMEAR IV, the aerosol activation diameter increased due to the decreased supersaturation with an increase in aerosol loading. In addition, at SMEAR IV during the fire episode, in situ measured cloud droplet number concentration (CDNC) increased by a factor of ∼7 as compared to non‐fire periods which was in good agreement with the satellite observations (MODIS, Terra). Results from this study show the importance of SE European fires for cloud properties and radiative forcing in high latitudes. Plain Language Summary: Wildfires are large sources of aerosol particles and affect human health and climate. Aerosols from fires are transported long distances in the atmosphere and affect the aerosol and cloud properties at places far from the actual sources. In this study, we measured the long‐range transported (LRT) fire air masses from south‐eastern (SE) Europe at a northern European and a high Arctic site. LRT fire emissions from SE Europe increase the aerosol number and mass loading in Finland and even in the high Arctic. Results show that the effect of fire emissions on aerosol hygroscopicity depends on the properties of both the LRT fires and the background aerosols at a given location. The cloud properties analysis in eastern Finland shows that despite high hygroscopicity and increased CCN activity, the aerosol activation diameter for clouds increased during the fire episode. This is due to the depletion of available water vapor in clouds due to the increased aerosol loading. Satellite observations show an increase in cloud droplet number concentration during the fire episode confirming the effect of LRT fires on cloud properties in eastern Finland. This study can improve the understanding of the effect of LRT fires on aerosol and cloud properties at remote locations. Key Points: Vegetation fires from southern Europe enhance aerosol and cloud condensation nuclei concentrations in northern Europe and the high ArcticA contrary trend in aerosol hygroscopicity is observed at these two locations during a strong fire episode as compared to non‐fire periodsCloud droplet number concentrations in liquid clouds show strong response to fire aerosol both in in situ and satellite observations [ABSTRACT FROM AUTHOR]

We use an ultrahigh‐resolution 15‐T Fourier transform ion cyclotron resonance mass spectrometer to elucidate the compositional changes in Arctic organic aerosols collected at Ny‐Ålesund, Svalbard, in May 2015. The Fourier transform ion cyclotron resonance mass spectrometer analysis of airborne organic matter provided information on the molecular compositions of aerosol particles collected during the Arctic spring period. The air mass transport history, combined with satellite‐derived geographical information and chlorophyll concentration data, revealed that the molecular compositions of organic aerosols drastically differed depending on the origin of the potential source region. The protein and lignin compound populations contributed more than 70% of the total intensity of assigned molecules when the air masses mainly passed over the ocean region. Interestingly, the intensity of microbe‐derived organics (protein and carbohydrate compounds) was positively correlated with the air mass exposure to phytoplankton biomass proxied as chlorophyll. Furthermore, the intensities of lignin and unsaturated hydrocarbon compounds, typically derived from terrestrial vegetation, increased with an increase in the advection time of the air mass over the ocean domain. These results suggest that the accumulation of dissolved biogenic organics in the Arctic Ocean possibly derived from both phytoplankton and terrestrial vegetation could significantly influence the chemical properties of Arctic organic aerosols during a productive spring period. The interpretation of molecular changes in organic aerosols using an ultrahigh‐resolution mass spectrometer could provide deep insight for understanding organic aerosols in the atmosphere over the Arctic and the relationship of organic aerosols with biogeochemical processes in terms of aerosol formation and environmental changes. Key Points: The molecular compositions of Arctic organic aerosols were identified using an ultrahigh‐resolution mass spectrometer (15T FT‐ICR MS)The molecular characteristics of Arctic organic aerosols showed distinct differences depending on their potential source originThe accumulation of biogenic organics in Arctic surface water could significantly influence the chemical properties of Arctic aerosols [ABSTRACT FROM AUTHOR]

Abstract. Primary marine aerosols are an important component of the climate system, especially in the remote marine environment. With diminishing sea-ice cover, better understanding of the role of sea spray aerosol on climate in the polar regions is required. As for Arctic Ocean water, laboratory experiments with NaCl water confirm that a few degrees change in the water temperature (Tw) gives a large change in the number of primary particles. Small particles with a dry diameter between 0.01 µm and 0.25 µm dominate the aerosol number density, but their relative dominance decreases with increasing water temperature from 0°C where they represent 85-90% of the total aerosol number to 10°C, where they represent 60°70% of the total aerosol number. This effect is most likely related to a change in physical properties and not to modification of sea water chemistry. A change of salinity between 15 g kg-1 and 35 g kg-1 did not influence the shape of a particle number size distribution. Although the magnitude of the size distribution for a water temperature change between 0 °C and 16°C changed, the shape did not. An experiment where succinic acid was added to a NaCl water solution showed, that the number concentration of particles with 0.010 µm < D > p < 4.5 µm decreased on average by 10% when the succinic acid concentration in NaCl water at a water temperature of 0°C was increased from 0 µmol L-1 to 94 µmol L-1. A shift to larger sizes in the particle number size distribution is observed from pure NaCl water to Arctic Ocean water. This is likely a consequence of organics and different inorganic salts present in Arctic Ocean water in addition to the NaCl. [ABSTRACT FROM AUTHOR]

During the ASTAR (Arctic Study of Tropospheric Aerosol and Radiation) campaign nucleation mode particles (4 to 13 nm) were quite frequently observed at altitudes below 4000 m. However, in the upper free troposphere, nucleation mode particles were only observed once, namely during the flight on 24 May 2004 (7000 m). To investigate if vertical motion were the reason for this difference that on one particular day nucleation mode particles were observed but not on the other days we employ a microphysical box model. The box model simulations were performed along air parcel trajectories calculated 6-d backwards based on European Center for Medium-Range Weather Forecasts (ECMWF) meteorological analyses using state parameters such as pressure and temperature in combination with additional parameters such as vertical stability. Box model simulations were performed for the 24 May where nucleation mode particles were observed (nucleation event) as well as for the days with measurements before and after (22 and 26 May) which are representative for no nucleation (non-nucleation event). A nucleation burst was simulated along all trajectories, however, in the majority of the simulations the nucleation rate was either too low or too high so that no nucleation mode particles were left at the time when the measurements were performed. Further, the simulation results could be divided into three cases. Thereby, we found that for case 1 the temperature was the only driving mechanism for the formation of new particles while for case 2 and 3 vertical motion have influenced the formation of new particles. The reason why nucleation mode particles were observed on 24 May, but not on the other days, can be explained by the conditions under which particle formation occurred. On 24 May the particle formation was caused by a slow updraft, while on the other two days the particle formation was caused by a fast updraft. [ABSTRACT FROM AUTHOR]

Number fluxes of particles with diameter larger than 10 nm were measured with the eddy covariance method over the Amazon rain forest during the wet season as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) campaign 2008. The primary goal was to investigate whether sources or sinks dominate the aerosol number flux in the tropical rain forest-atmosphere system. During the measurement campaign, from 12 March to 18 May, 60% of the particle fluxes pointed downward, which is a similar fraction to what has been observed over boreal forests. The net deposition flux prevailed even in the absolute cleanest atmospheric conditions during the campaign and therefore cannot be explained only by deposition of anthropogenic particles. The particle transfer velocity vt increased with increasing friction velocity and the relation is described by the equation vt=2.4×10-3×u* where u* is the friction velocity. Upward particle fluxes often appeared in the morning hours and seem to a large extent to be an effect of entrainment fluxes into a growing mixed layer rather than primary aerosol emission. In general, the number source of primary aerosol particles within the footprint area of the measurements was small, possibly because the measured particle number fluxes reflect mostly particles less than approximately 200 nm. This is an indication that the contribution of primary biogenic aerosol particles to the aerosol population in the Amazon boundary layer may be low in terms of number concentrations. However, the possibility of horizontal variations in primary aerosol emission over the Amazon rain forest cannot be ruled out. [ABSTRACT FROM AUTHOR]

The change in aerosol properties during the transition from the more polluted spring to the clean summer in the Arctic troposphere was studied. A six-year data set of observations from Ny-Ålesund on Svalbard, covering the months April through June, serve as the basis for the characterisation of this time period. In addition four-day-back trajectories were used to describe air mass histories. The observed transition in aerosol properties from an accumulation-mode dominated distribution to an Aitken-mode dominated distribution is discussed with respect to long-range transport and influences from natural and anthropogenic sources of aerosols and pertinent trace gases. Our study shows that the air-mass transport is an important factor modulating the physical and chemical properties observed. However, the air-mass transport cannot alone explain the annually repeated systematic and rather rapid change in aerosol properties, occurring within a limited time window of approximately 10 days. With a simplified phenomenological model, which delivers the nucleation potential for new-particle formation, we suggest that the rapid shift in aerosol microphysical properties between the Arctic spring and summer is mainly driven by the incoming solar radiation in concert with transport of precursor gases and changes in condensational sink. [ABSTRACT FROM AUTHOR]

We present measurements of aerosol physicochemical properties below 5 km altitude over the tropical rain forest and the marine boundary layer (MBL) obtained during the LBA-CLAIRE 1998 project. The MBL aerosol size distribution some 50-100 km of the coast of French Guyana and Suriname showed a bi-modal shape typical of aged and cloud processed aerosol. The average particle number density in the MBL was 383 cm-3. The daytime mixed layer height over the rain forest for undisturbed conditions was estimated to be between 1200-1500 m. During the morning hours the height of the mixed layer increased by 144- 180 m h-1. The median daytime aerosol number density in the mixed layer increased from 450 cm-3 in the morning to almost 800 cm-3 in the late afternoon. The evolution of the aerosol size distribution in the daytime mixed layer over the rain forest showed two distinct patterns. Between dawn and midday, the Aitken mode particle concentrations increased, whereas later during the day, a sharp increase of the accumulation mode aerosol number densities was observed, resulting in a doubling of the morning accumulation mode concentrations from 150 cm-3 to 300 cm-3. Potential sources of the Aitken mode particles are discussed here including the rapid growth of ultrafine aerosol particles formed aloft and subsequently entrained into the mixed layer, as well as the contribution of emissions from the tropical vegetation to Aitken mode number densities. The observed increase of the accumulation mode aerosol number densities is attributed to the combined effect of: the direct emissions of primary biogenic particles from the rain forest and aerosol in-cloud processing by shallow convective clouds. Based on the similarities among the number densities, the size distributions and the composition of the aerosol in the MBL and the nocturnal residual layer we propose that the air originating in the MBL is transported above the nocturnal mixed layer up to 300-400 km inland over the rain forest by night without significant processing. [ABSTRACT FROM AUTHOR]

While nucleation may represent one of the major processes responsible for the total aerosol number burden in the atmosphere, and especially at high altitude, new particle formation (NPF) events occurring in the upper part of the troposphere are poorly documented in the literature, particularly in the southern hemisphere. NPF events were detected and analyzed at the highest measurement site in the world, Chacaltaya (5240 m a.s.l.), Bolivia between January 1 and December 31 2012, using a Neutral Aerosol and Ion Spectrometer (NAIS) that detects clusters down to 0.4 nm. NPF frequency at Chacaltaya is one of the highest reported so far (63.9%) and shows a clear seasonal dependency with maximum up to 100% during the dry season. This high seasonality of the NPF events frequency was found to be likely linked to the presence of clouds in the vicinity of the station during the wet season. Multiple NPF events are seen on almost 50% of event days and can reach up to 6 events per day, increasing the potential of nucleation to be the major contributor to the particle number concentrations in the upper troposphere. Ion-induced nucleation (IIN) was 14.8% on average, which is higher than the IIN fractions reported for boundary layer stations. The median formation rate of 2 nm particles computed for first position events is increased during the dry season (1.90 cm −3 s −1 ) compared to the wet season (1.02 cm − 3 s −1 ), showing that events are more intense, on top of being more frequent during the dry season. On the contrary, particle growth rates (GRs) are on average enhanced during the wet season, which could be explained by higher amount of biogenic volatile organic compounds transported from the Amazon rainforest. The NPF events frequency is clearly enhanced when air masses originate from the oceanic sector, with a frequency of occurrence close to 1. However, based on the particle GRs, we calculate that particles most likely nucleate after the oceanic air masses reach the land and are presumably not originating from the marine free troposphere. The high frequency of NPF events, the occurrence of multiple events per day, and the relatively high formation rates observed at Chacaltaya imply that nucleation and growth are likely to be the major mechanism feeding the upper atmosphere with aerosol particles in this part of the continent. [ABSTRACT FROM AUTHOR]

To determine the direct and indirect effects of aerosols on climate, it is important to know the spatial and temporal variations in cloud condensation nuclei (CCN) concentrations. Although many types of CCN measurements are available, extensive CCN measurements are challenging because of the complexity and high operating cost, especially in remote areas. As aerosol optical depth (AOD) can be readily observed by remote sensing, many attempts have been made to estimate CCN concentrations from AOD. In this study, the CCN–AOD relationship is parameterized based on CCN ground measurements from the Zeppelin Observatory (78.91° N, 11.89° E, 474 m asl) in the Arctic region. The AOD measurements were obtained from the Ny-Ålesund site (78.923° N, 11.928° E) and Modern-Era Retrospective Analysis for Research and Applications, Version 2 reanalysis. Our results show a CCN–AOD correlation with a coefficient of determination R2 of 0.59. Three additional estimation models for CCN were presented based on the following data: (i) in situ aerosol chemical composition, (ii) in situ aerosol optical properties, and (iii) chemical composition of AOD obtained from reanalysis data. The results from the model using in situ aerosol optical properties reproduced the observed CCN concentration most efficiently, suggesting that the contribution of BC to CCN concentration should be considered along with that of sulfate. ● Relationship between CCN concentration and AOD was parameterized with statistically significant. ● AOD from MERRA-2 and MODIS were comparable with ground-based measurements of AOD. ● In-situ aerosol optical properties reproduce the CCN most efficiently. [ABSTRACT FROM AUTHOR]